Blood flow restriction as a post-exercise recovery strategy: A systematic review of the current status of the literature

The aim of this study was to systematically review the current literature on blood flow restriction (BFR) as a post-exercise recovery strategy. Experimental studies investigating the effect of BFR on recovery after exercise were included. Only studies meeting the following inclusion criteria were selected: (a) studies investigating about BFR as a post-exercise recovery strategy in athletes and healthy individuals; (b) the full text being available in English; (c) experimental research study design. Studies that exclusively analyzed BFR as a recovery strategy during the exercise (e.g., recovery strategy between bouts of exercise) were excluded. A literature review was conducted on the PubMed, Cochrane, and Web of Science electronic databases up until May 7th, 2023. The main findings were that (i) 9 studies investigated passive BFR as a post-exercise recovery strategy, which shows a significant lack of research in both team and individual sports (especially in female populations), and only 2 studies used active BFR protocols; (ii) although a high quality range of studies was observed, there were methodological limitations such as BFR interventions that were usually conducted after fatiguing protocols or fitness tests, which may not represent the real exercise (e.g., a sprint session of 6 sets of 50 m may induce muscle damage but it does not represent the demands of a team sport like rugby or soccer); (iii) there is a lack of consistency in BFR protocols (e.g., number of cycles or duration of the occlusion-reperfusion periods) for recovery; (iv) some studies showed beneficial effects while others found no positive or detrimental effects of BFR as a post-exercise recovery strategy in comparison with the control/SHAM group. In conclusion, only 11 studies investigated BFR as a post-exercise recovery strategy and there is not any significant amount of evidence in team or individual sports (especially in female populations). BFR could be a potential post-exercise recovery strategy, but practitioners should use caution when applying this method of recovery for their athletes and clients. In addition, it would be of interest for high performance-related practitioners to have a better understanding of the benefits of BFR interventions combined with either active or passive forms of exercise as a post-exercise recovery strategy.


Study design
A literature review was conducted on the PubMed, Cochrane, and Web of Science electronic databases up until May 7 th , 2023.This systematic review was reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [26] and considerations for systematic reviews in sport sciences [27].The following search strategy by title and abstract was designed: ["Blood flow restriction" OR "BFR" OR "occlusion" OR "ischemic conditioning" OR "ischemia-reperfusion"] AND ["sport*" OR "exercise" OR "athlete" OR "player" OR "resistance train*" OR "strength train*"] AND ["recovery" OR "cool-down" OR "post-conditioning" OR "post-session" OR "post-training" OR "post-workout" OR "post-match" OR "post-game" OR "post-exercise"].

Study selection
Only studies meeting the following inclusion criteria were selected: (a) studies investigating about BFR as a post-exercise recovery strategy in athletes and healthy individuals; (b) the full text being available in English; (c) experimental research study design.Studies that exclusively analyzed BFR as a recovery strategy during the exercise (e.g., recovery strategy between bouts of exercise) were excluded.Two independent reviewers selected the studies based on the inclusion and exclusion criteria.All references were stored in the Mendeley reference management system (Elsevier, Amsterdam, The Netherlands).Since duplicates were observed, these were removed.Then, the titles and abstracts were examined.Finally, articles were accessed full text and only studies meeting the inclusion criteria were included in the study.If there was any disagreement between the reviewers, a third collaborator was involved in the decision-making process.A graphical description of the selection process may be observed in Figure 1.

Data abstraction
The following data were extracted from each study by the reviewers: authors, year of publication, sample size, sample characteristics (e.g., age, gender, or body composition variables), type of exercise protocol or activity, characteristics of the BFR intervention, outcome variables (i.e., recovery-related parameters), and main findings regarding changes in these recovery-related variables after the BFR intervention.

Methodological quality assessment
The methodological quality of each study was assessed using the PEDro scale [28] if the study had an experimental design with randomly assigned experimental and control groups while the MINORS scale [29] was used for non-randomized trials.The PEDro scale consists of 11 items, which include: eligibility criteria, random allocation, concealed allocation, similarity at baseline, subject blinding, therapist blinding, assessor blinding, > 85% follow-up, intention-totreat analysis, between-group statistical comparison, and point and This has serious implications for sport performance practitioners since players might not achieve complete physical performance recovery during congested schedules [14].
In consequence, efficient recovery strategies are necessary and thus, the use of blood flow restriction (BFR) has been suggested as a strategy to accelerate recovery processes [15].BFR consists of restricting arterial inflow and completely restricting venous outflow due to the application of a tourniquet or inflatable cuff around the most proximal region of the working upper and/or lower limbs [16,17].The cuff's inflation generates a gradual compression of the vasculature underneath the cuff so the blood flow to structures distal the cuff is restricted, but it more severely affects venous outflow from under the cuff that is proposed to also impede venous return [16].This compression implies a hypoxia condition within the muscle tissue [18,19].
From a practical perspective, recent studies recommended the use of BFR combined with various forms of exercise (e.g., resistance exercise, aerobic exercise, or passively) even though other factors such as intensity, volume, restriction time, amount of cuff pressure, size, and cuff material) need to be considered [16,20,21].
Although a better knowledge of the underlying mechanisms through which BFR may positively impact post-exercise recovery, the rationale behind the use of this technique may be related to both metabolic and vascular pathways [22].BFR interventions were initially developed in order to decrease the damage caused to internal organs by ischemia and reperfusion [22,23] but it may be a useful strategy for performance purposes [22].For example, the primary mechanisms by which BFR is thought to stimulate muscle growth may include metabolic accumulation which leads to a subsequent increase in anabolic growth factors, fast-twitch fiber recruitment, and more protein synthesis through the mammalian target of rapamycin (mTOR) pathway [24].In addition, nitric oxide synthase-1 (NOS-1), heat shock proteins, and myostatin have also been shown to be affected by an occlusion stimulus [24].Also, a different study found that the recovery of functional outcomes after applying BFR post-exercise may be due to a lower decrease in creatine kinase and muscle soreness [25].
However, the benefits of BFR as a strategy to accelerate post-exercise recovery processes are still unclear.Although another review was published in 2020, an update is necessary given the large number of sports performance-related practitioners that are currently using BFR for recovery [15].In addition, one might wonder if the conclusion of such review (i.e., effective intervention to accelerate performance recovery) is strong enough from a practical application perspective based on the characteristics of the studies that were included.Given that research on the effects of BFR as a recovery strategy is scarce, the aim of this study was to systematically review the current literature on BFR as a recovery strategy in order to investigate the effect of BFR protocols, which were performed after exercise in athletes and healthy individuals, on recovery-related parameters.

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Blood Flow Restriction and Post-Exercise Recovery variability measures [28].The first item was not included to calculate the total PEDro score as suggested by previous research, so the maximum score was 10 points (low quality: <3; moderate quality: 4-5; high quality: 6-10) [30][31][32].When it comes to the MINORS scale, it consists of 12 items (i.e., a clearly stated aim, inclusion of consecutive patients, prospective collection of data, endpoints appropriate to the aim of the study, unbiased assessment of the study endpoint, follow-up period appropriate to the aim of the study, loss to follow up less than 5%, prospective calculation of the study size, adequate control group, contemporary groups, baseline equivalence of groups, and adequate statistical analysis), being the last 4 items specifically for comparative studies [29].Each item was scored on a scale of 0-2 (0: not reported; 1: reported but inadequate; 2: reported and adequate) for a total score of 24 points (low quality: <8; moderate quality: 9-16; high quality: 17-24) [32].Any hesitation in the scoring process was resolved by consensus between two independent researchers.

Study selection
A total of 1947 studies were identified following the search strategy.
Duplicates were removed and a total of 656 titles and abstracts were examined.Of these, 46 were selected for full-text screening.Since 35 studies did not meet the inclusion criteria, 11 studies were selected for the study (Figure 1).
The cuffs for the control/SHAM groups were usually inflated between 15-and 20-mmHg.Specifically, the BFR protocols consisted of a total of 3 cycles of 5-minute occlusion followed by 5-minute reperfusion or 2 cycles of 3-minute occlusion followed by 3-minute reperfusion [34,39,40,42].
Another study [37]   Participants adopted a supine position immediately after the exercise protocol.Unilateral occlusion cuff (BJ Dare Medical Equipment, China), which was placed on the proximal portion of the leg.The cuff had a pneumatic bag along its inner surface which was connected to a pressure gauge that was manually inflated to either 15 mmHg (control group) or 220 mmHg (BFR group) for 3 min.
The cuff was alternated with the contralateral leg for an additional period of 3 min.This cycle was repeated twice for a total of 12 min, so both legs had BFR intervention for 6 min per leg.
From Participants adopted a supine position and bilateral arterial occlusion cuffs were placed on the proximal portion of the thigh (14.5 cm width; Delfi Medical Innovations, Vancouver, Canada).The inflatable cuffs were connected to a pressure gauge and were automatically inflated to 220 mmHg (BFR group) for 5 min followed by 5 min reperfusion.The pressure for the SHAM group was set at 20 mmHg.This procedure was repeated three times (i.e., 15

Main findings related to BFR interventions in team sports
Two studies analyzed the effect of BFR on the recovery process in semiprofessional soccer players [33] and youth players [36], and two BFR interventions were conducted on academy and amateur rugby union players [34,35].The youth soccer players did not get beneficial effects from the active BFR protocol for recovery of jumping ability or perceived wellness (i.e., Hooper index).In fact, the BFR group, which wore the cuffs during the recovery session, had increased ratings of perceived exertion right after the session [36].However, the study with semiprofessional soccer players found that the BFR group got to attenuate the increases in muscle damage markers and muscle soreness in comparison with the placebo group [33].Also, the BFR group got to attenuate the decrease of squat jump and CMJ height in 24 hours [33].In addition, BFR could accelerate post-exercise recovery of sprint performance and maximal voluntary contraction of 5-minute occlusion followed by 5-minute reperfusion and 5 cycles of 2-minute occlusion followed by 2-minute reperfusion.Also, two studies included active BFR protocols for recovery (e.g., BFR during the recovery session 24 hours post-match or active protocols including 30 repetitions of biceps curls without external load, followed by 3 sets of 15 repetitions).
Regarding the outcome variables that could be associated with the recovery status, the studies analyzed parameters related to muscle soreness, perceived recovery status, perceived effort, neuromuscular function (e.g., strength, speed, or power), and physiological markers (e.g., creatine kinase, salivary cortisol, lactate dehydrogenase, or C-reactive protein).
Finally, the methodological quality assessment showed that the PEDro scores for the randomized trials [25,[33][34][35][36][37][39][40][41][42] were ~7.6 (range 6-10), which may be considered as high quality.Also, only one of the studies followed a non-randomized protocol, so the MINORS scale was used and the score was 21 out 24, which represents a high quality score [40].there were also large positive effects of the BFR on recovery of eccentric peak power and eccentric peak acceleration.Also, the BFR had a beneficial effect in comparison with the control condition on the mean concentric and eccentric peak velocity during CMJ.Moreover, the total power generated in the leg press test showed better results for the BFR group 24 hours post intervention.In addition, the BFR group recovered at a better rate in comparison with the control group when the cumulative 10 m and 40 m sprint times were analyzed 24 hours after the intervention.
On the other hand, there were likely detrimental effects on mean eccentric peak velocity and acceleration in the CMJ and mean eccentric peak power in the squat jump immediately after the BFR intervention [40].Other immediate effects were generally unclear or trivial [40].Likely detrimental effects of the BFR were found in the change in rate of recovery on the mean concentric work generated in the squat jump 24 hours after the intervention [40].Other delayed effects were generally unclear [40].
Finally, another study used an active BFR protocol [41] in which each arm was randomly assigned to either the blood flow restriction or the SHAM treatment (0 mmHg).The BFR intervention was performed 5 min and 24 hours after the fatiguing protocol, but the results indicated that this type of intervention did not improve recovery [41].

DISCUSSION
The purpose of this study was to review the current literature on the use of BFR as a post-exercise recovery strategy.The main findings were that (i) only 9 studies investigated BFR as a post-exercise passive recovery strategy, which shows a significant lack of research in both team and individual sports (especially in female populations), and only 2 studies used active BFR protocols; (ii) although a high quality range of studies was observed, there were methodological limitations such as BFR interventions that were usually conducted after fatiguing protocols or fitness tests (e.g., incremental cycling test or sprint tests), which may not represent the real exercise (e.g., a sprint session of 6 sets of 50 m may induce muscle damage but it does not represent the demands of a team sport like rugby or soccer); (iii) there is a lack of consistency in BFR protocols (e.g., number of cycles or duration of the occlusion-reperfusion periods) for recovery; (iv) some studies showed beneficial effects while others found no positive or detrimental effects of passive BFR as a post-exercise recovery strategy in comparison with the control/SHAM groups.
This review observed that there is lack of research in team and individual sports.Only 2 studies were conducted in team sports like soccer or rugby, and only 1 study for individual sports like judo or cycling, so it is difficult to draw solid conclusions about the benefits of BFR as a post-exercise recovery strategy.In addition, the methodology of these investigations has several limitations such as the sample characteristics (e.g., lack of research on multiple populations like quadriceps [33].Nevertheless, BFR had no effect on perceived recovery, neuromuscular or physiological markers after the exercise protocol and BFR in the academy or amateur rugby players [34,35].

Main findings related to BFR interventions in individual sports
One study analyzed the effect of BFR on the recovery process in cyclists with ~3.2 years of training experience [37] and another BFR intervention was conducted on elite judo athletes [38].Both studies found beneficial effects of BFR as a recovery strategy compared to the SHAM groups.For example, BFR prevented a decrease in performance 24 hours after the incremental cycling test, which may be due to a late effect of the BFR [37].Creatine Kinase and muscle soreness were similar between the groups of cyclists (baseline and 24 hours post exercise) [37].However, perception of recovery scores showed that the post-exercise ischemic conditioning groups felt more tired 24 hours after BFR [37].
In judo athletes, the BFR group had a decrease in heart rate at 30-and 60-minutes during recovery [38].In addition, CMJ performance in the BFR group was better at 60 minutes compared to the control group [38].Moreover, the BFR group reported lower muscle soreness than the control group [38].

Main findings related to BFR interventions in recreationally active individuals
The use of BFR as a recovery strategy has been investigated in other recreational activities (i.e., trained/active/healthy individuals) [25,[39][40][41][42].
One of the studies showed that BFR interventions may accelerate the recovery process since a return to pre strength levels 24 hours earlier than the SHAM group was observed [25].This recovery of functional outcomes may be due to a decrease in the inflammatory response observed after strenuous eccentric exercise because of reduced creatine kinase and muscle soreness [25].In this line, a different study found that BFR interventions had a significant but marginal effect on mitigating perceptual quadriceps muscle soreness ratings 24 hours after the drop jump fatiguing protocol; however, there was no effect on muscle strength [42].
However, another study concluded that the occlusion and sequential intermittent pneumatic compression groups did not further improve recovery of muscular performance after a fatiguing exercise protocol compared to a passive control group [39].Perceived recovery and muscle soreness were not significantly different in comparison with the control group [39].Nonetheless, participants reported that they preferred the "novel recovery interventions" [39].
A third study on recreationally active individuals observed that BFR intervention had positive and negative effects on specific variables related to the neuromuscular function [40].On the one hand, BFR had a positive effect on the average squat jump height right after the intervention compared to the control group [40].Specifically, most positive effects of BFR were found 24 hours after the intervention [40].For example, there was a likely positive effect on the professional and amateur athletes, or female samples), assessor/therapist/participant blinding (perhaps due to the nature of the intervention), or the exercise protocols that have been used to induce fatigue.
For instance, one of the studies in soccer players used the Loughborough intermittent shuttle running test, which is a field test that simulates the activity pattern of soccer [43] However, actions such as running backwards, jumping, or time in possession of ball that are not included and some of these activities along with tackles and sudden changes of direction require eccentric contractions, which may increase the neuromuscular demands imposed by match in comparison with Loughborough intermittent shuttle test [44].In addition, this kind of data collections is not contextualized to elite level scenarios, in which soccer players may experience successive matches (e.g., less than 96 hours between games) in congested calendars [12].In con- and different recovery protocols have been included, which may explain why some studies got positive effects and others did not.
Although BFR interventions were usually conducted with the participant on a supine position and on the proximal portion of the thigh, this literature review also found that there is a lack of consistency in BFR protocols for recovery.For example, the cuffs for the experimental groups were usually inflated at 220 mmHg [25,39,40] or 50 mmHg above systolic blood pressure [33,37,38].In this regard, a previous review suggested that absolute tourniquet pressures from 200 to 250 mmHg have been often used regardless of body size [45].One of the studies explained that they use a novel technology for BFR which determined the individual personal tourniquet pressure as a percentage of limb occlusion pressure, where venous blood flow was completely restricted [42].There were 2 studies using 60% of individually calculated pressures [34,36] and another study using 80% of individual's resting arterial occlusion pressure [41].This shows that to date, there has been a lack of standardization of pressure for BFR and recovery.A previous study suggested that it might be recommended to establish pressures based on measurements of arterial occlusion pressure, with pressures ranging from 40 to 80% of arterial occlusion pressure [16].The reason is that the arterial occlusion pressure is related to a wide range of individual characteristics (e.g., size of the limb, individual's blood pressure, or tourniquet shape, width, and length) [16,46,47].However, this range of 40 to 80% of arterial occlusion pressure would make more sense for BFR with exercise and the total occlusion pressure (i.e., 100%) may be more closely related to BFR for recovery.
In addition, a previous systematic review observed that some of the studies analyzing the effect of BFR on recovery did not clearly describe the characteristics of the participants and expressed concern about the statistical analysis [15].Consequently, these limitations have a significant effect on understanding the benefits of BFR as a post-exercise recovery strategy.
Another main finding of this literature review was that some studies observed beneficial effects of BFR on the recovery process while others found no positive or detrimental effects of BFR as a post-exercise recovery strategy in comparison with the control/SHAM group.
A total of 5 studies highlighted some beneficial effects of BFR on recovery [25,33,37,38,40], but unclear or detrimental effects were observed in 6 studies [34-36, 39, 41, 42].In this regard, a previous investigation concluded that BFR interventions could lead to overall faster performance recovery, lower creatine kinase increase, and lower muscle soreness over 24 hours [15].In addition, it has been suggested that the effectiveness of this intervention may be more specific to low/moderate fitness level individuals [15] and the mechanisms that explain the benefits of BFR for improving recovery and performance are unclear, but likely involve changes in both vascular and metabolic pathways [48].This inconsistency among studies might be also due to the fact that recovery may be understood from multiple perspectives (e.g., changes in physiological parameters such as creatine kinase or modifications of perceived recovery and neuromuscular performance-related variables).
This review has several limitations to acknowledge.For example, the lack of studies, heterogenous methodologies, and the discrepancy between the results on the benefits of BFR as a post-exercise recovery strategy make it difficult to draw solid conclusions.For example, specification of cuff width and material should be included by each study as these variables may impact the effectiveness of BFR interventions [16].Also, studies on BFR for recovery were mainly focused on passive strategies, so future research is not only necessary in this direction but also considering active strategies [16].In this regard, the authors from a previous study recommended using BFR combined with various forms of exercise (e.g., resistance or aerobic) [16].

CONCLUSIONS
BFR could be a potential post-exercise recovery strategy, but practitioners should use caution when applying this a method of recovery for their athletes and clients.Only 11 studies investigated BFR as a post-exercise recovery strategy and there is not any significant amount of evidence in team or individual sports (especially in female populations).Some studies showed beneficial effects while others found unclear or detrimental effects of passive BFR as a post-exercise recovery strategy in comparison with the control/SHAM groups.Also, a lack of consistency in BFR protocols (e.g., number of cycles or duration of the occlusion-reperfusion periods) for recovery was observed.
In addition, this study has implications for practice and future research.Although limited data are available, the BFR protocols that Biology of Sport, Vol.41 No3, 2024 199 Blood Flow Restriction and Post-Exercise Recovery to be analyzed as well.For example, the timing of the intervention in high performance sports is very important, especially when there is travel involved after the competition.Also, further research is neeed to understand if it is likely that a single bout of restriction can improve recovery for the next days based on the initial change in blood flow.Furthermore, knowing the estimated duration of the effect of a BFR intervention would help understand the evolution of the recovery process.Nonetheless, it is necessary to gain a better understanding of the mechanisms of BFR for recovery.For instance, if the potential beneficial effects of BFR on exercise performance and recovery can be linked to post occlusion increases in blood flow associated with elevated adenosine levels and activation of ATP-sensitive potassium channels, which may lead to increased blood flow and improved skeletal muscle contractile function [39,40].In this regard, future research will be necessary to understand the effects of BFR on recovery markers.

Table 1 .
Characteristics of included studies.
(12)cles for each leg with 5 min of reperfusion for each BFR episode.The BFR group achieved a mean pressure of 180(12)mmHg while the control group was set at 20 mmHg.

Table 1 .
min of BFR and 15 min of reperfusion).Continue.
Note: BFR = Blood flow restriction; CMJ = Countermovement jump; RM = repetition maximum; RPE = rating of perceived exertion in both scales were those related to random allocation, concealed allocation, and assessor/therapist/participant blinding.

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of recovery of maximal power production (W) in the squat jump in comparison with the control group.Regarding the squat jump, The criteria with the lowest scores Biology of Sport, Vol.